1. Field of the Invention
This invention relates to encoding apparatus and to methods associated with these. More particularly, the invention relates to application of acoustic methods to encoding keyboards. The present application is an improvement of the acoustic methods disclosed in copending applications Ser. No. 853,778 filed Nov. 21, 1977, now abandoned, and Ser. No. 892,814 filed Apr. 3, 1978 now U.S. Pat. No. 4,258,356, both having the same assignee. To the extent appropriate to the present invention, the disclosures of the above-identified copending applications are incorporated herein by reference.
2. Description of the Prior Art
Encoding apparatus for use with many and varied types of equipment have long been known. Yet, there is a continued search for low cost encoders having high reliability. Apparatus of that type based on detection of acoustic wave fronts and specifically applied to keyboards have been briefly disclosed in IBM Technical Disclosure Bulletins--e.g. Arosenius, Vol. 14, No. 10, March 1972 and Lisk, Vol. 29, No. 1, June 1977--and specific approaches using long bars or "rods" are more fully disclosed in the above-mentioned copending applications and in British patent No. 1,386,070. The technique disclosed in the first copending application Ser. No. 853,778 (referred to hereinafter as "prior application I", for brevity) and in the above-identified British patent is based on determination of an elapsed time while the technique of the second copending application (referred to hereinafter as "prior application II"), involves selective striking of different combinations of several rectangular bars according to the presence or absence of tabs at corresponding points on each of the bars.
While a considerable improvement over known art, the keyboard using the first technique has encountered significant limitations when considerations of mass production have been introduced, even though still simpler and lower in cost than prior art keyboards. It has been found that an encoding apparatus based upon elapsed time determinations on acoustic waves as disclosed in prior application I is also sensitive to variations in the acoustic properties of the material from one batch to another and, in some instances, even along the length of the stock from which the bars are made. The accumulation of tolerances has been such that realization of the full economies inherent in the technique has not been possible for a full (64 key) typewriter keyboard. Likewise, while the keyboard using the second technique presents advantages in terms of striker performance, this is not significant enough to outweigh the costs (even in high manufacturing quantities) of the additional transducers required. Accordingly, there is need for an encoding apparatus of the acoustic type which is less sensitive to accumulation of tolerances though still economical.